The present invention relates to a toner image fixing device which has a novel construction for thermally fixing a toner image developed on a sheet-like carrier, e.g., a sheet of recording paper.
In a conventional image recording device, recording medium forms thereon a toner image and transfer it onto a common paper sheet which is subjected to fixing the toner image thereon and then is delivered out of the device. Generally, the toner image is fixed on a paper sheet by fusing.
A fixing device used in a conventional toner image fixing device comprises a rotatably mounted heating roller made of aluminum drum coated with fluorocarbon resin (e.g., PTFE: polytetrafluoroethylene sold under the trade name "Teflon") which is well-releasable from toner and a pressure roller coated with silicone rubber. The heating roller and the pressure roller are disposed as pressed against each other to form therebetween a contact portion (nip) utilizing elastic deformation of the pressure roller. While a sheet carrying a toner image developed with toner thereon passes through the nip, the toner image is heated and fixed by fusing onto the sheet. A heater consisting of, e.g., a halogen lamp is mounted in the heating roller to heat the latter at a specified temperature necessary for fusing toner of the toner image on the sheet. And a separating finger pressed at its head against the external cylindrical surface of the heating roller to separate the sheet from the heating roller.
In the above-mentioned fixing device, the heating roller is made of an aluminum pipe having wall thickness of 1.0 mm to several millimeters and outer diameter of 20 to 60 mm. A toner image developed with toner on the paper sheet is fixed by fusing while the sheet passes through a nip (contacting portion) between the pressure roller and the heating roller heated at a specified temperature by the heater axially mounted therein.
Another example of a toner image fixing device that is different from the above-mentioned device is proposed in Japanese laying-open patent publications Nos. 59-58766 and 63-313182, wherein a belt being an endless film of 10 to 50 microns in thickness made of heat-resistant material (e.g., polyamide) envelops therein a heater supporting member which supports a heater having a resistance on a ceramic substrate in such a manner that the heater may be in contact with an internal surface of the endless belt. A pressure roller disposed in opposite to the heater through the endless belt is pressed against an external surface of the endless belt to form a nip portion through which a paper sheet having a developed toner image passes being subjected to fixture the toner thereon by fusing.
As described previously, the conventional toner image fixing device uses the heating roller having a thickwall of 1.0 to several millimeters in radial direction, which, therefore, shall be previously heated by conduction heat to a specified working temperature of its surface for a warm-up time of several seconds to several minutes. The long warm-up time of the heating roller deteriorates the controllability of the device as well as increases the power consumption. To put a paper sheet into contact with the heating roller, it is needed to use the pressure roller having a metal core coated with silicone rubber, which is expensive in itself and increases a manufacturing cost of the conventional device.
In comparison with the above-mentioned device, the other conventional device uses a thin- film type belt to be heated and, therefore, can save its warm-up time and reduce power consumption required. However, this device also has to use the pressure roller for putting the toner-image carrying paper sheet into close contact with the rotating endless belt, that irrevocably leads to increasing the manufacturing cost of the device. The device must be provided with means for driving the endless belt, (e.g., a driving roller and a driven roller), that may not only complicate the construction of the device but also increase its manufacturing cost.
Each of the conventional art devices uses a fixing member of roll-like form, which can not freely change a nip width and fixing power and, furthermore, may cause curling of recording paper along its body surface.
The necessary fixing power depends upon thickness of recording paper. The thicker recording paper is, the more it absorbs heat, i.e., the less heat is applied for fusing toner on the recording paper if heating temperature (for fixing) is constant. This may result in insufficient fusing the toner onto a thick paper sheet. Furthermore, some paper materials may not easily allow fixation of the toner image developed thereon. In such cases it is effective to selectively change, for instance, increase a nip portion (i.e., contacting surface) of a member for fixing by heat toner image on recording paper, that may increase fixing efficiency and improve the fixing condition of the toner image. For this purpose, pressing force of a pressing member to be applied to a heated member (e.g., a heating roller) is increased to form a wider nipping surface. However, increasing the pressing force to the heating roller may not sufficiently increase the nip width and may result in forcing the paper to be curled.
In the conventional devices, the contact surface of the roller, which comes into contact with a toner-image carrying paper sheet, is heated up to a temperature necessary for fusing toner and has a narrow limited nip width. Therefore, fixing is effected by heating the toner image to the boundary surface of the paper sheet. In this case, an upper part of the toner layer contacted to the heating roller may be heated to abnormally high temperature, reduce cohesive force and transfer to the heating roller surface (i.e., so-called high-temperature offset of toner occurs). On the contrary, when the fixing temperature is adjusted to a relatively low value, toner may keep well-cohesive power but be poor in adhesion to the paper sheet causing so-called low-temperature offset due-to insufficient fusion of the toner on the paper sheet.
In short, the paper sheet is heated at the same temperature during contacting with the heating roller since a whole cylindrical surface of the roller is evenly heated up to a specified temperature. Therefore, the toner may be over-heated causing the high-temperature offset. On the contrary, the low-temperature toner offset may occur due to insufficient fusing if the roller surface temperature is adjusted to a relatively low temperature. For this reason, the conventional devices require high-accuracy temperature control as well as application of offset preventive liquid to the surface of the heating roller to be used.
Even temperature distribution over a whole nip (contact) portion may cause the above-mentioned toner offset. The nip width may be changed by changing pressing force of the pressure roller but this may intensify curling of the paper sheet. Namely, the recording paper in heated state may be easily curled according to a curvature of the heating roller.
The curled paper may jam in the passage when it enters into the image-forming portion again for printing the other side (for two-side printing) or the same side (for double printing). To avoid this, it is necessary to provide a straightener at the downstream side of the fixing device.
In addition, the paper sheet after fixing the toner image thereon may also be in close contact with the heating roller and requires forcibly separation therefrom by a separating finger. To make the paper sheet depart by itself from the heating roller surface without using the separating finger, it is necessary to increase a curvature of the roller to such an extent that the front end of the paper sheet may not follow up the roller. The roller can have a larger curvature by reducing its diameter. In this case, the roller shall have a diameter of not more than 20 mm. This is, of course, accompanied by increasing a degree of curling of the paper sheet. Furthermore, the heating roller of not more than 20 mm in diameter may be hard and expensive to manufacture and may not posses sufficient durability.
As mentioned above, arrangement of a roller-like-formed member (i.e., a heating roller) in a heating portion may inevitably cause curling of recording paper. Namely, the heating roller usually has a constant nip and can not change fixing power by changing its nip width. Pressing force of the heating roller may be increased but can not attain a sufficient change of its nip width resulting in considerable curling of recording paper. Natural separation of recording paper from the heating roller may be realized by using a heating roller having an increased curvature and a reduced diameter. This roller, however, is more expensive to manufacture and may increase curling of recording paper. Consequently, one solution encounters another problem, that is, curling of the recording paper may be increased.